Recently electronic devices have been more miniaturized and portable electronics devices including note-type personal computers have been widely used. A small, surface-mountable acceleration sensor with quality are more needed in order to certify the reliability of such electronic devices against shock.
A high-density hard disc can be taken as an example. If the disc is shocked during writing operation, the head is displaced and, as a result, the data cannot be written or the head itself can be damaged. In order to avoid such problems, it is necessary to detect shock to the head, and then stop writing or move the head to a safe position.
Demands for a shock detecting device acceleration sensor for an airbag apparatus are also increased so as to protect a driver from the shock caused by a car collision.
It is also needed to install a configuration in an apparatus which detects shock applied to a portable device and avoids failure or malfunction of the device due to the shock, or records the shock. Therefore, the needs for a small acceleration sensor used for such a device have been also increased.
Acceleration sensors employing piezoelectric materials such as piezoelectric ceramics have been well-known. Such an acceleration sensor can realize a high detection sensitivity by using the electromechanical conversion characteristics of the piezoelectric materials. A piezoelectric acceleration sensor outputs forces caused by acceleration or vibration after converting these forces into voltage by the piezoelectric effect. One example of such an acceleration sensor uses a cantilever structure rectangular bimorph electromechanical transducer as disclosed in Unexamined Japanese Patent Application (Tokkai-Hei) 2-248086. As shown in FIG. 26 of this application, a bimorph electromechanical transducer 50 using the piezoelectric effect is produced by fastening piezoelectric ceramics (51a, 51b) formed with electrodes (52a, 52b) with an adhesive 53 (e.g. epoxy resin). The cantilever structure shown in FIG. 27 is formed by adhering and fixing an end of the electromechanical transducer 50 to a fixing portion 55 with, for example, a conductive adhesive 54. Such a cantilever structure electromechanical transducer having low resonance frequency is used for measurement of acceleration having relatively low frequency components. In order to measure acceleration in a high frequency region, another type of bimorph electromechanical transducer 50, both of whose ends are fixed to fixing portions 55 with, for example, a conductive adhesive 54, is used (see FIG. 28). The resonance frequency can be relatively raised by fixing both ends of the electromechanical transducer (a structure clamped at both ends).
An acceleration sensor is formed by setting the electromechanical transducer 50 in a package while holding the fixing portion 55 to the inner wall of the package. Electric charge generated at the electrodes (52a, 52b) of the electromechanical transducer 50 is conducted out to outer electrodes via, for example, the conductive adhesive 54.
As mentioned above, adhesives including an epoxy resin are used to adhere the piezoelectric ceramics of conventional acceleration sensors. Young's modulus of the epoxy resin is 200.times.10.sup.-12 m.sup.2 /N, which is bigger than that of the piezoelectric ceramic (150.times.10-.sup.12 m.sup.2 /N), so the epoxy resin absorbs the distortion of the electromechanical transducer due to acceleration, and as a result, the sensitivity deteriorates. In addition to that, it is still difficult to adhere piezoelectric ceramics while keeping the thickness of the adhering layer uniform, therefore, the characteristics of the electromechanical transducer will vary.
The resonance frequency of a rectangular bimorph electromechanical transducer should be stable in order to make its sensitivity stable. For this purpose, the electromechanical transducer should be fixed firmly. Actually, however, its metallic supporters or portions supported or fixed by fixing portions will be displaced because of stress generated mechanically or by temperature variation. For instance, if an electromechanical transducer is fixed by using adhesives, the fixing positions will change depending on the adhesive-application range, and thus its resonance frequency will vary. In another case, the fixing condition of the electromechanical transducer will depend on the temperature, so the stable fixing condition is not easily maintained.
In case electromechanical transducers are respectively produced and then set in packages, handling becomes difficult in the producing steps. As a result, the acceleration sensor cannot be miniaturized and quantity production becomes difficult.
The piezoelectric ceramic is produced by mixing and firing several kinds of materials, so, its material constants vary compared to that of a single crystal material. Therefore, sensitivity and capacitance considerably vary.
An acceleration sensor employing piezoelectric ceramics is also used to detect shock on a portable device. Such a device, however, considerably varies in its sensitivity, the standard acceleration range which is set to protect apparatuses from failure tends to be large, and, thus, precise shock detection becomes difficult. Due to the capacitance variation, it is difficult to design a circuit which is connected to the acceleration sensor in order to amplify electric signals generated from acceleration, and, thus, the amplifier degree of the circuit becomes irregular. As a result, the output signal considerably varies, and thus the acceleration sensor cannot reliably be used for shock detecting.